Information flows
from left to right. DNA is what genes are made
of. Proteins do
a lot of stuff, like make structures (muscle fibers, for example),
are receptors on the surfaces of cells, and are enzymes, that
catalyze chemical reactions of biological importance (eg: metabolic
enzymes digest sugars and liberate energy, releasing CO2
in the process).

Irreducible Complexity?

If you are worried
about how life originated, this creates a bit of a problem:

You need proteins
to copy the DNA.

You need DNA to
code for (and ultimately to make) proteins.

So which came
first?

This is an example
of what Creationists have termed "irreducible
complexity"; there seems no way out of this conundrum except to
invoke divine intervention (or creation).

A Seminal Discovery

RNA is much like
DNA, except a bit less stable. (DNA is a better long-term
information storage devise, and RNA, though less stable, is easier
to get information out of, so DNA is first "transcribed" into RNA,
which shares a similar four-letter alphabet, and is then
"translated" into proteins.)

So if RNA can be
both an information-storage molecule, and an enzyme (or "ribozyme"), it is a
good candidate for the first self-replicating molecule, and it gets
us out of the above-mentioned Catch-22.

Theories of the
origin of
life that start out with an assumption that the first
self-replicating molecules were ribozymes are called RNA World hypotheses.
This isn't the only idea for how life originated, but it is
probably the most favored one currently.

Another Major Problem

But there's a
catch.

In order to copy
RNA, fragments or monomers (individual building blocks) that have
5'-triphosphates must
be ligated together. This is true for modern (protein-based)
polymerases, and
is also the most likely mechanism by which a ribozyme
self-replicase in an RNA World might function. Yet no one has found
a modern natural ribozyme that can do this reaction.

Artificial Evolution

RNA test tube (in vitro) evolution and selection has however
enabled several research groups to discover RNA sequences that can
in fact catalyze the required chemical reaction for 5'-triphosphate
RNA fragment ligation, and one group has even produced a functional
RNA polymerase ribozyme. This provides a Proof of Principle that
RNA is capable of such feats, and absent time travel, this is
likely the best we will be able to do.

My coauthor,
Michael Robertson, evolved a ligase ribozyme that performs the
desired 5'-3' RNA
assembly reaction. He called this the L1 ligase. To better
understand the details of how this ribozyme folds into a structure
that permits it to catalyze this fundamental reaction, he and I
have solved its X-ray crystal
structure.